The present invention relates to apparatus and methods for the acoustic analysis of bone, and more particularly to apparatus and methods for accomplishing bone measurement using signal processing techniques.
The prior art is rich with approaches to measurement of bone characteristics using acoustic and other methods with a view to identifying patients in need of treatment for osteoporosis. Many acoustic techniques utilize a first transducer to provide an acoustic signal, typically at ultrasonic frequencies, to the subject from a first external location and a second transducer at a second external location disposed on the opposite side of the bone of interest to receive the signal transmitted by the first transducer through the bone and intervening soft tissue. (The transducers are typically coupled to the subject through a suitable fluid, such as water.) Under one approach, there is determined the rate of Broadband Ultrasound Attenuation (BUA) in the range of approximately 300 to 700 kHz. The BUA is determined by measurement of the attenuation at a plurality of frequencies and then fitting the measurements to a suitable linear logarithmic-amplitude versus frequency scale. However, as an indicator of osteoporotic bone, BUA does not provide a desirable level of specificity and sensitivity.
The present invention provides, in some embodiments, enhanced specificity and sensitivity in determining an index of porosity and non-connectivity of a bone by utilizing a number of surprising discoveries, including: (i) spectral estimation of a received ultrasound signal in bone is advantageously nonlinear and time variant; (ii) utilization of received signal information (such as phase) that is lost in BUA analysis permits more accurate assessment of bone condition; (iii) the use of transducers approximating a point source and a point receiver provide enhanced performance and flexibility in placement; and (iv) significant portions of the acoustic energy lost in attenuation in direct transmission through the bone can be measured by suitable placement of a third transducer in a position distinct from the path of direct transmission.
In accordance with a preferred embodiment of the invention, there is provided an apparatus for externally determining in a vertebrate subject an index of porosity and non-connectivity of a bone. The embodiment has first and second transducers and a mounting arrangement for mounting the transducers in spaced relationship with respect to the bone. A signal generator, in communication with the first transducer, causes the first transducer to produce acoustic signals, having energy distributed over a frequency range, that are propagated into the subject and received by the second transducer along a path that includes the bone. Finally, the embodiment has a signal processor, in communication with the second transducer, for providing a measurement that is a function of at least one of spectral or temporal components of the signal received by the second transducer. The function is selected for its ability to minimize differences among successive measurements taken of the same individual and to maximize differences in measurements taken of different individuals, so that the measurement relates to the porosity and non-connectivity of the bone. In a further embodiment, the function is a weighted sum of spectral components of the signal received by the second transducer, and the weights are selected for their ability to minimize differences among successive measurements taken of the same individual and to maximize differences in measurements taken of different individuals.
In related embodiments, the a signal processor provides a single measurement that is a function of at least one of spectral or temporal components of a portion, up to the whole amount thereof, of the signal received by the second transducer, such measurement being other than a log-linear slope estimation based on Fourier spectral information. In a preferred embodiment a selected one or both of the transducers employs a vibrating element that is sufficiently small as to cause the selected transducer, if driven by the signal generator, to produce an acoustical output, into the body part, that is substantially like that of a point source. The function may include any or a combination of the following:
a weighted sum of spectral components of a portion of the signal received by the second transducer;
a measure of the shape of the Hilbert envelope of a portion of the signal received by the second transducer;
a measure of the shape of anautoregressive moving average spectral estimation function of a portion of the signal received by the second transducer;
a measure of the variability of the Hilbert frequency function of a portion of the signal received by the second transducer;
a measure of the average Hilbert frequency function of an early portion of the signal received by the second transducer;
a weighted sum of spectral components, determined using a short-time Fourier transform, and determined at successive intervals, of the signal received by the second transducer, wherein the successive weighted sums associated with successive intervals are themselves formed into a weighted sum;
a measure of the group delay of a portion of the signal received by the second transducer; and
a measure of the normalized ratio of narrow-band energy to broad-band energy of a portion of the signal received by the second transducer.
In a further embodiment there may be provided a third transducer, affixed to the mounting arrangement, for receiving, along a second path that is distinct from, and which may be transverse to, the first path, acoustic energy supplied by the first transducer.
In a preferred embodiment, the invention includes one or more transducers utilizing a piezoelectric crystal element that has an aspect ratio that is substantially less than 5:1 and substantially greater than 1:5, less than approximately 2:1 and greater than approximately 1:2, and preferably approximately 1.5:1.
In a further embodiment, the invention provides a system for externally providing a measurement in a vertebrate subject of the characteristic behavior of an acoustic wave in a bone disposed within a body part. This embodiment has first and second transducers and a mounting means for mounting the transducers in spaced relationship with respect to the bone, all of which are contained in a first assembly. Also provided are signal excitation means for causing the first transducer to produce an acoustic waveform that is propagated into the subject and received by the second transducer along a path that includes the bone and characteristic determination means for determining a characteristic of the behavior of the waveform along the path. A display for displaying outputs of the characteristic determination means is contained in a second assembly that is hand-holdable and permits the measurement to be taken while holding the second assembly in one hand of the user. The signal excitation means and the characteristic determination means are collectively contained within the first and second assemblies. In further embodiments, the characteristic determination means includes a signal processor for providing a single measurement that is a function of at least one of spectral or temporal components of a portion, up to the whole amount thereof, of the signal received by the second transducer. The first assembly may be realized as an appliance for removable engagement with the foot of a subject. The appliance has a base having a surface for receiving the sole of a foot having a longitudinal axis; a cradle rigidly attached to the base, for receiving subject""s foot and ankle, and disposed in a direction transverse to the base, a yoke for supporting the transducers in spaced relationship with respect to the bone; and a member for mounting the yoke in moveable relationship to the base so as to permit joint two-dimensional motion of the transducers over regions of the heel including the calcaneus. The yoke and member constitute the mounting means. The member is a backplate hingedly attached at one end to the base along a first hinge axis generally transverse to the longitudinal axis; and the other end of the backplate is hingedly attached, along a second hinge axis generally parallel to the first hinge axis, to the yoke.
A further embodiment of the invention is the appliance itself, which may include a control module, physically mounted to at least one of the base, cradle, yoke or member. The module has a first set of data ports coupled to the transducers, a second set of data ports for coupling to (i) a signal excitation means for causing a first one of the transducers to produce an acoustic waveform that is propagated into the subject and received by a second one of the transducers along a path that includes the bone and (ii) characteristic determination means for determining a characteristic of the behavior of the waveform along the path. The control module includes a microprocessor for controlling motors for causing displacement of the yoke and therefore the transducers over the calcaneus. The module has a control port over which the microprocessor receives control signals for the motors from a master microprocessor. Preferably, a selected one or both of the transducers employs a vibrating element that is sufficiently small as to cause the transducer, if driven by the signal generator, to produce an acoustical output, into the body part, that is substantially like that of a point source, and each of the transducers employs a resonating element that has a diameter less than 0.5 cm.
In a related embodiment, the invention provides an apparatus for externally determining in a vertebrate subject an index of porosity and non-connectivity of a bone disposed within a body part. The apparatus has first and second transducers, a mounting arrangement for mounting the transducers in spaced relationship with respect to the bone; a signal generator, in communication with the first transducer, for causing the first transducer to produce an acoustic signal, having energy distributed over a frequency range, that is propagated into the subject and received by the second transducer along a first path that includes the bone; and a signal processor, in communication with the second transducer, for providing a single measurement that is a function of at least one of spectral or temporal components of a portion, up to the whole amount thereof, of the signal received by the second transducer. A selected one or both of the transducers employs a vibrating element that is sufficiently small as to cause the selected transducer, if driven by the signal generator, to produce an acoustical output, into the body part, that is substantially like that of a point source; preferably each of the transducers employs a resonating element that has a diameter of less than 1 cm.
A further embodiment provides an apparatus for externally determining in a vertebrate subject proximity of an ultrasonic signal path to an edge of a bone disposed within a body part. The apparatus of this embodiment has first and second transducers; a mounting arrangement for mounting the transducers in spaced relationship with respect to the bone; a signal generator, in communication with the first transducer, for causing the first transducer to produce an acoustic signal, having energy distributed over a frequency range, that is propagated into the subject and received by the second transducer along a first path that includes the bone; and a signal processor, in communication with the second transducer, for determining a measure of the relative proportion of high frequency energy in relation to low frequency energy of the signal received by the second transducer. The proportion determined is an indication of the proximity of the first path to an edge of the bone. Preferably, each of the transducers employs a vibrating element that is sufficiently small as to cause each transducer, if driven by the signal generator, to produce an acoustical output, into the body part, that is substantially like that of a point source.
In a related embodiment there is provided a method for externally determining in a vertebrate subject proximity of an ultrasonic signal path to an edge of a bone disposed within a body part. The method includes providing first and second transducers; mounting the transducers in spaced relationship with respect to the bone; utilizing a signal generator, in communication with the first transducer, to cause the first transducer to produce an acoustic signal, having energy distributed over a frequency range, that is propagated into the subject and received by the second transducer along a first path that includes the bone; and processing the signal received by the second transducer so as to determine a measure of the relative proportion of high frequency energy in relation to low frequency energy of the signal received by the second transducer. Again, the proportion is an indication of the proximity of the first path to an edge of the bone. Preferably in step(a) each of the transducers employs a vibrating element that is sufficiently small as to cause each transducer, if driven by the signal generator, to produce an acoustical output, into the body part, that is substantially like that of a point source.
Another embodiment of the invention provides a method for externally determining in a vertebrate subject an index of porosity and non-connectivity of a bone disposed within a body part. The method includes:
(a) providing first and second transducers, wherein a selected one or both of the transducers employs a vibrating element that is sufficiently small as to cause the selected transducer, if driven by the signal generator, to produce an acoustical output, into the body part, that is substantially like that of a point source;
(b) mounting the transducers in spaced relationship with respect to the bone;
(c) utilizing a signal generator, in communication with the first transducer, to cause the first transducer to produce an acoustic signal, having energy distributed over a frequency range, that is propagated into the subject and received by the second transducer along a first path that includes the bone; and
(d) processing the signal received by the second transducer so as to provide a single measurement that is a function of at least one of spectral or temporal components of a portion, up to the whole amount thereof, of the signal received by the second transducer.